To ascertain whether the addition of exercise thallium scintigraphy (TS) to standard exercise electrocardiography (ECG) would enhance the prediction of future cardiac events in clinically healthy subjects, we performed both ECG and TS with maximal treadmill exercise in 409 volunteers (aged 40-92 years) from the BLSA who were free of cardiac disease by history, physical examination, and resting ECG. Over a mean followup interval of 4.6 year, 42 coronary events were recorded. The predictive value for a coronary event in the 23 subjects with both ischemic ECG and abnormal TS was 48%. By logistic regression analysis, the combination of ischemic ECG and abnormal TS predicted a 6.3 fold relative risk of a subsequent coronary event, independent of conventional risk factors (age, sex, smoking status, hypertension, total cholesterol). Thus, the combination of ischemic ECG and abnormal TS in totally asymptomatic subjects is a potent predictor of future coronary events. Among 696 apparently healthy subjects from the BLSA who have undergone serial maximal exercise treadmill testing since 1977, the risk of subsequent cardiac events was compared in those whose initial test was positive (Group I) versus those converting form a negative to a positive rest (Group II) over a 5.7 year mean followup. Thus is asymptomatic subjects, serial conversion from a negative to a positive exercise ECG is of no greater predictive value for a future coronary event than an initially positive response.